The present invention relates to a four-port network for the separation of four signal paths associated with two polarizations in each of two frequency bands, whose input waveguide, in which the signals of both frequency bands and both polarizations are able to propagate, is connected with the feed horn of a microwave antenna, and which is provided with a circuit branching arrangement for separating the frequencies and polarizations. More particularly the present invention relates to an improved arrangement for coupling the four-port network to the microwave antenna feed horn in a system wherein the antenna is provided with a monopulse tracking system for the antenna.
Microwave antennas are frequently utilized to transmit and receive signals from two frequency bands and two polarizations. Feeder systems permitting such multiple utilization of the antenna are known by the name of four-port network, system filter or quadruplexer. The state of the art includes four-port networks designed for two linearly orthogonally polarized frequency bands (see, for example, Federal Republic of Germany DE-AS No. 2,443,166 and corresponding U.S. Pat. No. 3,978,434), and those designed for two circularly orthogonally polarized frequency bands (see, for example, DE-OS No. 2,932,626 and corresponding U.S. Pat. No. 4,344,048, and DE-OS No. 2,703,878 and corresponding U.S. Pat. No. 4,319,206).
Such four-port networks are used, for example, in ground station antenna for satellite radio operating in the 4/6 GHz range. Particularly in connection with satellite radio, methods and apparatus are required which permit tracking of the ground station antenna with the associated satellite. Customarily this is done with the aid of the known monopulse method according to which antenna deviation signals are derived from the radiation characteristics of one or a plurality of higher order waveguide modes. Such monopulse methods are described in great detail in, for example, German Pat. No. 2,135,611 and corresponding U.S. Pat. No. 3,758,880 and in German Pat. No. 2,212,996 and corresponding U.S. Pat. No. 3,864,683.
In satellite radio, the frequency band (4 GHz) of the downward or received signal generally lies far below the frequency band (6 GHz) of the upward or transmitted signal. The monopulse device of the ground station antenna needs the downward signal (received signal) to derive the deviation signals. This generally involves the problem that the higher order wave modes required to otain the deviation signal are able to exist only in the throat of the antenna feed horn. The reason for this is that the higher order wave modes are reflected in the tapering throat of the antenna feed horn which permits only the fundamental mode to pass. Coupling in the higher order modes can therefore take place only within the conical horn throat. Devices for coupling the higher order modes into a waveguide and particularly into a conical antenna feed horn are known as mode couplers, e.g. see German Pat. No. 2,135,611 and corresponding U.S. Pat. No. 3,758,880, German Pat. No. 2,212,996 and corresponding U.S. Pat. No. 3,864,683, DE-AS No. 2,460,552 and corresponding U.S. Pat. No. 3,964,070, and German Pat. No. 2,608,092 and corresponding U.S. Pat. No. 4,048,592. These publications show that the mode couplers are rather costly structures. Moreover, there exists the danger that the coupling in of the higher order modes by means of a prior art mode coupler directly at the antenna feed horn (see DE-AS No. 2,460,552 and corresponding U.S. Pat. No. 3,964,070) will excite undesirable modes, as well as causing useful signal components to be coupled out.